[FEATURE] Filter event data prior to triggering a source / workflow invocation

Today, I receive a large number of events from some source that I need to filter out before running my workflow. Often the events contain no data / missing fields, and aren't relevant for my workflow.

Currently, these events trigger an invocation on Pipedream, and therefore contribute to my usage. I'd like a way to filter these events prior to running my workflow, in a way that doesn't trigger an invocation.